What screw should I use to bolt refer plate to floor?

[ramblinChet]

I am preparing to bolt a steel plate (32.0" x 18.5" x 1/16") down to the floor of my FWC and I would like to hear some suggestions on what would be the best wood screw to use. This steel plate will hold my 80 liter National Luna refrigerator and the plate has six 1/4" holes about the perimeter used to fasten it to the deck.

My first consideration was a #8 screw but since much of the loading would a shear type load with only some being a withdraw load I thought moving up to a #10 would be a good idea. So, six #10 with 1/8" pilot holes. Your thoughts?

Now onto the length...it appears that the plywood decking in my FWC is 5/8" thick and I know the steel plate I am bolting down in 1/16" thick. From what I can gather it would be best to sink the screw at least 1/2 to 2/3 of the thickness of the plywood.

So, shooting for 1/2 of the depth would require a #10 screw of 3/8" length while 2/3 depth would require a #10 of 1/2" length. Your thoughts?

For this application I am not sure of the screw head type matters that much although I am always open to suggestions. If I had the ability to remove my camper I believe it would be best to bolt through the floor but that is not an option right now.

 

[rando]

If you may need to remove the plate in the future, then a threaded insert like these maybe a good option. Lots of holding power, and you won't blow out the threads in the wood removing/reinstalling the mounting plate. Wood screws work well the first time they are installed, then progressively worse every subsequent time.

 

[fish more]

X2 Agree with rando/then you can use counter sunk bolts instead of wood srews, much nore secure

 

[ramblinChet]

Oh wow, those are very nice and I can certainly see the value in them. Probably the best option for my application - much more robust compared to a small wood screw at shallow depth into the soft wood. Very nice and thank you!

Has anyone used these previously in a similar way? Any helpful hints on installation?

 

[rando]

I use dozens of these type of threaded inserts for work holding on my CNC Router, they work well even in MDF which is notoriously bad for holding threads. Not much of a trick to installation - drill the correct size hole (11mm diameter for the ones linked above), at least as deep as required (deeper is OK). For just installing a few you don't need the install tool, for those specific ones an 8mm hex/allen socket will work, but check the specs on whatever size you end up with.

I would recommend you use metric inserts/fasteners, so you can use the same tools you use for your truck. It drives me nuts that all the camper and aftermarket folks insist on using inch hardware, now you need to carry two sets of tools.

 

[Dirtroadsavant]

While we’re on the subject…. I am getting ready to tear out our dinette and replace with a bench that will better serve our use. The small woodscrews that hold together the original dinette tend to vibrate loose on fun roads. Would these threaded inserts offer a solution if I used them in the new build? If so, Is threadlock overkill, or just the ticket?

 

[ramblinChet]

I am not sure how threadlocker would work in relation to the threaded inserts but if you would like to experiment you might want to contact the technical line at Henkel and ask their opinion regarding what your expectations are, what materials are being bonded, diameters, temperatures, etc.

For similar applications I have use LOCTITE 222MS Purple Threadlocker. Download the Technical Data Sheet PDF for additional information.

 

[rando]

These threaded inserts are really designed for applications where you both want to be able to easily remove and reinstall the screws and also don't have access to the back of the wood or or the back side needs to be pretty. You don't want to use thread lock on the inserts, you will just unscrew the insert, defeating the purpose of the insert.

If you have access to the backside and it isn't visible, then T-Nuts like these are a more secure option and you could use thread lock on these as the tangs will keep them from spinning.

If you just want to replace the current screws with something with more holding power in plywood, you can use a 'blunt' screws like these, which are short but fat with well defined threads the grip along the entire length. I have had luck squirting a little wood glue in the holes to help stop screws from loosening up.

If you are starting from scratch on new cabinets/benches, then consider pocket hole screws and glue.

 

[ntsqd]

If you do have access to the backside then I'd suggest using the t-nuts. I'm linking mcmaster.com, but they are far from being the only source for these:

https://www.mcmaster.com/tee-nuts/

Can even get them in stainless steel, or metric, or both!

Given that the holes in the plate are 1/4" I'd use nothing smaller than a #10 or a 5mm, and would prefer to use either 1/4" or 6mm (with the 6mm's being my top choice for a range of reasons).

 

[Beach]

#10x1/2 truss head screw. Predrill holes and put a dab of 5200 on the screws to prevent them from loosening/backing out

 

[ntsqd]

What will the fridge weigh when full?

Figure that you'll have to be able to hold 3 times that weight during the slam-bang from that pot-hole or dog-launcher that you didn't see.
Divide that weight by the number of possible screws (6 as I recall) to get the loading for each screw. Consider that it isn't just the screw's tensile strength being tested by these events. It is how the screw is anchored that is also being tested.